some portion of its routing table only to its
neighbors. Distance vector algorithms perform routing decisions
based upon information provided by neighboring routers.
Distance vector protocols use fewer system resources but can
suffer from slow convergence and may use metrics that do not
scale well to larger systems. Distance vector protocols are
based on finding the distance (number of hops) and vector
(direction) to any link on the internetwork. The algorithms
involve passing copies of a complete routing table from router
to router on a periodic basis. This type of routing protocol
requires that each router simply inform its neighbors of its
routing table. For each network path, the receiving routers
pick the neighbor advertising the lowest cost and then adds
this entry into their routing table for readvertisement. RIP
and IGRP are common distance vector routing protocols. Web
Links Distance Vector Routing
http://www.cs.uwa.edu.au/~chris/ cnet/aricstewart/
Content 6.3 Routing Protocols
Overview 6.3.6 Link-state Link-state
algorithms (also known as shortest path first algorithms) flood
routing information to all routers in the internetwork that
creates a map of the entire network. Each router sends packets
to all its neighbors. These packets contain descriptions of the
network or networks to which the router is linked. The routers
assemble all the information into a complete view of the
internetwork topology to calculate the shortest path to all
known sites on the network. It then generates routing tables
showing the best path for any destination on the network. Once
converged, link state protocols use small update packets, which
contain only changes rather than copies of the entire routing
table. Update packets are passed across the network in
event-triggered updates, so convergence is fast.Because they
converge more quickly than distance vector protocols,
link-state algorithms are less prone to routing loops.
Link-state protocols are also less prone to routing errors, but
they use more system resources. Link-state protocols,
therefore, can be more expensive to implement and support.
However, they are generally more scalable than distance vector
protocols. When a network link changes state, a notification
called a link-state advertisement (LSA) is flooded throughout
the network. All the routers note the change and adjust their
routes accordingly. This method is more reliable, easier to
debug, and less bandwidth-intensive than distance vector. OSPF
and IS-IS are examples of link-state routing protocols.
Content Summary An understanding of the following key
points should have been achieved: - A router will not
forward a packet without a route to a destination
network.
- Network administrators manually configure
static routes.
- Default routes are special static
routes that provide routers with gateways of last resort.
- Static and default routes are configured using the ip
route command.
- Static and default route
configuration can be verified using the show ip route,
ping, and traceroute commands.
- Verify
and troubleshoot static and default routes
- Routing
protocols
- Autonomous systems
- Purpose of
routing protocols and autonomous systems
- The classes
of routing protocols
- Distance vector routing protocol
features and examples
- Link-state protocol features and
examples
- Route determination
- Routing
configuration
- Routing protocols (RIP, IGRP, OSPF,
EIGRP, BGP)
- Autonomous systems and IGP versus
EGP
- Distance vector routing
- Link-state
routing